1. Field of the Invention
The present invention relates to the structure of a battery module having a plurality of cells electrically connected, and a container for a battery module. Particularly, the present invention relates to the structure of a battery module improved in vibration resistance and heat resistance and a container thereof suitable as a battery module for electric vehicles having a large lithium secondary battery, for example, electrically connected.
2. Description of the Background Art
A battery module having a group of a plurality of unit cells electrically connected is conventionally known. Various types of battery modules with different voltages and capacities in addition to the supplemental function of displaying the remaining capacity and the like are produced in order to correspond to various applications, and are now available on the market as a portable power supply.
The unit cell forming the battery module has a structure in which a collector piece is welded to the current collector. Upon exertion of vibration, the collector piece fixed by welding will receive stress to result in fraction of the collector piece itself or the weld portion thereof.
Electric vehicles are recently known as the new application for a battery module. Since the battery module for an electric vehicle is generally used in the situation where vibration is constantly exerted, there is the need of improving the poor vibration resistance arising from the internal structure of the unit cell. There is also the heat radiation demand for a structure that discharges outside the heat generated from the unit cell in order to prevent increase in the temperature of the unit cell that becomes the cause of degrading the unit cell performance.
Japanese Patent Laying-Open No. 6-203823, for example, discloses a battery developed to accommodate the aforementioned need of improving the vibration resistant property. According to the technique disclosed in this publication, the structure of the interior of the unit cell is modified. The power generation member is held elastically within the unit cell by connecting the current collector and the collector piece by means of compression. The problem of stress being exerted on the collector piece that is fixed by welding when vibration occurs to result in fracture is solved. However, this method by compression connection requires the component and the procedure of fabricating a unit cell to be partially modified since modification of the internal structure of the unit cell is indispensable.
Although vibration resistance is improved in the unit cell according to the compression connection method, resistance at the connecting section increases since the connection between the collector piece and the current collector is not electrically sufficient in comparison to the connection by the conventional welding method. As a result, the voltage drop is great when a great current is discharged. Problems will be encountered in the battery characteristics such as reduction in the cell capacity. Thus, there was a problem that a battery module using unit cells according to such a method is degraded in performance than a battery module using conventional cells.
As another approach of accommodating the demand for vibration resistance, a structure of buffering the vibration by securing the unit cell in a container using rubber is generally known. Although vibration resistance is improved by the secure structure using rubber, degradation in the performance of the unit cell caused by the heat generated from the charge/discharge of the unit cells in the battery module cannot be suppressed since the heat dissipation ability of rubber is low.
Therefore, when improvement in vibration resistance is required in a battery module of the conventional type, the unit cell per se had to be modified. Furthermore, when a great current is to be output from this cell, the voltage drop will increase to reduce the capacity of the unit cell since the internal resistance is high. There was a problem that a battery module using such unit cells has low performance.
As a technique of improving the heat radiation property of a battery module, Japanese Patent Laying-Open No. 9-86188 discloses the art of cooling a unit cell that generates heat in response to the charge/discharge by providing a hole in the container for cooling.
However, for a battery module formed of a plurality of unit cells according to such a conventional structure, the difference in temperature between the outer side unit cell located outside in the proximity of the hole for cooling and the inner side unit cell located further away from the hole could not be suppressed sufficiently. Difference in temperature between the unit cells in a battery module will result in different conditions for the charge and discharge in respective unit cells. The capacity of each of the unit cells in the battery module will differ to degrade the essential performance of the battery module.